The D.C. United midfielder is set to be sidelined until mid-October after suffering the injury in training Wednesday.
D.C. United's place in the postseason seems safe, but the club's bid for the Supporters' Shield and top playoff seed just took a substantial hit.
Midfielder Chris Rolfe will be sidelined for six weeks after suffering a fractured left forearm in training Wednesday. The 31-year-old, who is third on the team with six goals and second with six assists, underwent surgery Wednesday night.
United (14-8-4) is first in the Eastern Conference, four points ahead of Sporting Kansas City while boasting a game in hand. The club trails the Seattle Sounders by five points in the Supporters' Shield race.